The control of bleeding during surgery accounts for a major portion of the time involved in an operation. In particular, bleeding that occurs when tissue is incised or severed can obscure the surgeon's vision, prolong the operation, and adversely effect the precision of cutting. Blood loss from surgical-cutting may require blood infusion, thereby increasing the risk of harm to the patient.
It is known to use electrosurgical techniques to reduce bleeding from incised tissue. By using electrosurgical techniques, a high frequency or radio frequency current can be passed through the patient's tissue between two electrodes for both cutting and causing hemostasis in tissue. The current passing through the tissue causes joulean (ohmic) heating of the tissue as a function of the current density and the resistance of the tissue, and denatures the tissue proteins to form a coagulum that seals the bleeding sites.
Monopolar electrosurgical devices employ a small electrode at the end of a handle in the surgeon's hand and a large electrode plate beneath and in contact with the patient. Only one of the two electrodes required to complete the electrical circuit is manipulated by the surgeon and placed on or near the tissue being operated on. The other electrode is the large plate in contact with the patient. The electrosurgery power supply impresses high frequency voltage spikes of thousands of volts between these two electrodes, sufficient to cause an electric arcing from the small operating electrode the surgeon holds to the most proximate tissues, then through the patient to the large electrode plate contacting the patient. In the patient, the electrical current becomes converted to heat; hottest in the tissues immediately adjacent to the small hand-held electrode where the currents are most concentrated.
In bipolar electrosurgical devices, two electrodes are closely spaced together and have the same surface area in contact with the tissue. The current flow is thus locally confined to the tissue that is disposed between and electrically connects the electrodes.
Because of the dangers associated with the use of electrical instruments during surgery, special properties are required of the instruments. For example, it is known to use parylene as a conformal coating for the electrosurgical instruments. It is unique as a coating because of its ability to provide ultra-thin films and conform to substrates of varied geometrical shapes and irregularities. Parylene has excellent chemical resistance and can be used at relatively high temperatures.
Parylene is a generic term applied to the family of unsubstituted and substituted poly(p-xylylenes). The polymers can be homopolymers or co-polymers depending on whether they are derived from one particular dimer or a mixture of different cyclic dimers. In general, these cyclic dimers have the following structural formula, ##STR1## wherein R is a substituent on the aromatic ring, x and y are each integers from 0 to 3, inclusive, R' is H, Cl or F. Typical R groups include hydrogen, hydrocarbon, oxyhydrocarbon, thiohydrocarbon, hydroxyl, halogen, nitro, nitrile, amine and mercapto groups. After these cyclic dimers are pyrolized, they may be in the form of diradicals having the structures ##STR2## or moieties having the tetraene or quinoid structures represented by the formulas, ##STR3## According to the patent literature, it may be that the tetraene or quinoid structure is the dominant structure which results when the cyclic dimer is pyrolyzed, but that the monomer polymerizes as though it were in the diradical form. As previously discussed, the preparation of these cyclic paraxylylene dimers and their pyrolytic cleavage and subsequent condensation to form copolymers and homopolymers is well known and described in the patent literature, particularly U.S. Pat. Nos. 3,149,175, No. 3,342,754, No. 3,288,728, No. 3,246,627, No. 3,301,707, No. 3,600,216 and No. 4,291,245 all of which patents are incorporated by reference herein.
Parylene coatings can be made from commercially available starting materials such as Parylene N, Parylene C, and Parylene D.
Parylene N coatings are produced by vaporizing di(p-xylylene) dimer, pryolyzing the vapor to produce p-xylylene free radicals, and condensing a polymer from the vapor onto a substrate that is maintained at a relatively low temperature, typically ambient or below ambient. Parylene N is derived from di(p-xylylene), while parylene C is derived from di(monochloro-p-xylylene), and parylene D is derived from di(dichloro-p-xylylene).
Although parylenes have generally advantageous electrical, chemical resistance and moisture barrier properties, it has been found that these polymers do not adhere well to many substrate surfaces, particularly under wet conditions. Although these polymers are quite resistant to liquid water under most conditions, they are subject to penetration by water vapor which may condense at the interface between the parylene film and the substrate, forming liquid water which tends to delaminate the film from the substrate. Vapor deposited parylene films are also generally quite crystalline and are subject to cracking which may also create undesirable paths for penetration of moisture to the substrate surface.
It is also known to use ceramic coatings on electrosurgical instruments because of the unique properties of ceramic. Unlike metals, which have relatively high thermal conductivity, ceramic coatings are noted for their thermal barrier properties. However, ceramic coatings tend to be porous and develop cracks making such coatings less than ideal for electrosurgical applications.
It is an object of the present invention to provide an improved dielectric coating for electrosurgical implements.
It is a further object of the present invention to provide such a dielectric coating that reduces the likelihood of capacitative build-up of current during electrosurgery.